A brief discussion on the development of photovoltaic energy storage DC microgrid monitoring system

1. Introduction

Microgrid refers to a small power generation and distribution system composed of multiple energy sources. The purpose of microgrid is to realize the flexible and efficient application of distributed power sources and solve the problem of grid connection of a large number of distributed power sources in various forms. The development and extension of microgrid can fully promote the large-scale access of distributed power sources and renewable energy, realize the highly reliable supply of multiple energy forms to loads, and is an effective way to realize active distribution network, making the transition from traditional power grid to smart grid.

In view of the fact that the energy management platform of the existing power system monitoring and data acquisition system has poor flexibility and long debugging cycle, and lacks the ability to measure and control building energy consumption parameters such as ambient temperature and water vapor, it cannot support renewable energy buildings and community cogeneration systems to achieve optimized energy management and refined energy efficiency analysis. This project will develop a photovoltaic energy storage DC smart microgrid monitoring system.

2. Technical background

近几年，全球范围应对气候变化进程显著加速，将气候变化问题作为21世纪人类面临的重大挑战之一成为全球共识。2020年9月，我国提出“力争2030年前实现碳达峰、2060年前实现碳中和”的战略决策。应对碳中和领域的挑战，在绿色转型中实现共同发展，已经成为国内国际面临的重要共同任务。据国际能源署（IEA）数据显示，2018年，中国碳排放（折算为CO2，下同）约为100亿t，其中，建筑运行约为21亿t，占21%，建筑业约为18亿t，占18%。同西方发达相比，我国建筑运行碳排放占比偏低，但建筑材料和建造碳排放占比偏高。2022年4月1日，住房和城乡建设部出台《建筑节能与可再生能源通用技术规范》，对建筑节能和太阳能、地热能、空气能与建筑的结合应用提出了更具体的要求，将逐步提升至近零能耗建筑。由于应用了各种类型的能源形式，依靠常规的电网和化石能源的建筑能源系统正在向多种能源综合利用模式发展。同时，多种储电、储冷、储热技术得到广泛应用，使得能源系统更加复杂多样。靠人工控制和简单的自动化控制难以满足建筑综合能源系统可靠、高效运行的需要。光伏储能直流智能微电网监测系统已经成为国内外的研究热点。

Siemens SpectrumPower microgrid management system and EnergyIPDEOP cloud platform can complete the unified management of thermal power multi-energy complementarity and power load. Schneider Electric launched a new generation of digital energy efficiency management platform EMS+, which realizes equipment visualization management, equipment preventive maintenance and BIM3D modeling; Japanese companies such as Panasonic and Hitachi launched community-level and household-level general energy management platforms such as CEMS and HEMS, realizing the coordination of energy supply and load energy consumption. Foreign software is generally large in scale and weak in plasticity, making it difficult to adapt to the diverse needs of domestic building energy systems.

Domestic building monitoring and energy management software is mainly derived from conventional power monitoring software, such as KingView, Power Control, MCGS, Ruier, Jiekong and other configuration software. Its main functions are still at the stage of data collection and statistical analysis. If load control and energy efficiency management are required, customized development is required, which is costly and time-consuming. Compared with foreign integrated energy management platforms, domestic platforms are in their infancy, with relatively simple functions, and it is difficult to achieve coordinated operation of multiple energy sources.

1. Technical Solution

In order to solve the problem of intelligent management and control of community or industrial park-level energy systems, a photovoltaic energy storage DC smart microgrid monitoring system is planned to be developed. The system consists of two parts: a hardware controller based on embedded technology and a main control system. It is based on providing customized services for various energy utilization scenarios such as commercial parks, modern communities or new towns, and improving the safety, reliability and economy of energy use. The overall technical solution and functions are as follows:

(1) Develop edge controllers based on embedded technology, whose main functions include communication, data acquisition, system protection, intelligent control and energy management.

(2) Develop an energy management master control system that not only has the monitoring, protection, data storage, event recording, and human-computer interaction functions of the SCADA system, but also has intelligent energy management functions.

(3) Develop software modules with different control and energy management functions that can be freely combined and called to achieve multi-objective optimization control.

(4) The system's hardware equipment and software programs are highly compatible, supporting ports and software programs of various communication protocols, and can manage various energy forms such as photovoltaics, wind power, natural gas, electric heating, electric energy storage, and thermal energy storage.

3.1 System Framework Construction

The photovoltaic energy storage DC smart microgrid monitoring system adopts the "cloud-edge-end" collaborative energy management system architecture. Through the collaborative method of "local computing + cloud optimization", it realizes the online optimization and upgrade of the system and reduces the dependence on network communication. The system architecture is shown in Figure 1.

The system framework is divided into the distribution network dispatching layer, the microgrid centralized control layer, and the local control layer. The local control layer includes power generation, energy storage system, load and AC/AC control parts. The microgrid centralized control layer includes control, which is composed of monitoring units and statistical analysis. The monitoring unit includes three parts: power generation, energy storage, and load according to different monitoring objects. Power generation monitors its key parameters such as voltage, current, and power; energy storage monitoring includes voltage, current, power, and charge status; load monitoring includes type, power, and power consumption; and statistical analysis uses a variety of display forms to analyze the operating status and decision-making processing of each part. The protection part protects energy storage, power supply, and users respectively, and the metering part settles electricity bills through meters. The distribution network dispatching layer has a dispatching system, which dispatches energy according to the results of statistical analysis to achieve optimal utilization of energy.

3.2 Software and Hardware Design

3.2.1 Energy Storage System

The energy storage system consists of two parts: energy storage battery and inverter. The energy storage battery can be a lead-acid battery, lithium iron phosphate battery, or flywheel energy storage system; the function of the inverter is to control the energy storage part and perform AC/DC conversion. The inverter has two working modes: power closed-loop operation and voltage closed-loop operation, and operates in grid-connected and off-grid states respectively. The DC side voltage of the inverter is the working voltage of the energy storage system, and the AC side voltage is usually 380V or 400V bus voltage. The energy storage system is also equipped with a battery management system for real-time detection of the voltage, current, temperature and other parameters of the energy storage unit, and through high-precision remaining power and battery health estimation, it evaluates the discharge current of the battery and uploads monitoring parameters.

3.2.2 Energy Management System

The energy management system studies low-power wireless transmission technology and modular networking communication technology for cold, heat, electricity and gas physical quantities, and develops embedded edge controllers. The main functions are communication, data acquisition, system protection, as well as intelligent control and energy management functions.

3.2.3 Microgrid Monitoring and Management System

The microgrid monitoring and management system is based on the energy management cloud platform design technology of the browser and server (B/S) architecture mode. In addition to the monitoring, protection, data storage, event recording, human-computer interaction and other functions of the data acquisition and supervision control (SCADA) system, it also has intelligent energy management functions. The main control system is highly compatible and scalable, and can form a customized system architecture and realize monitoring and protection functions according to different application scenarios.

3.3 Performance parameters

The system is highly compatible and scalable, and can be customized according to different application scenarios. The system also provides corresponding control management strategies and monitoring and protection functions. In this process, it not only supports the scheduling and allocation of various energy sources, but also considers the cross-coupling utilization of different energy forms such as electric heat conversion and electric cooling conversion.

The main parameters of the system are shown in Table 1.

4. System functions

4.1. Real-time monitoring

The system has a friendly human-machine interface, which can display the operating status of the energy storage cabinet, and monitor PCS, BMS and environmental parameter information in real time, such as electrical parameters, temperature, humidity, etc. It can also display relevant information such as faults, alarms, and benefits in real time.

4.2. Equipment monitoring

The system can monitor the operating status and operation mode of PCS, BMS, electric meters, air conditioners, fire protection, dehumidifiers and other equipment in real time.

PCS monitoring: meet the parameter and limit settings of the energy storage converter; set the operation mode; collect and display the voltage, current, power and charge and discharge parameters of the AC and DC sides of the energy storage converter; and monitor the PCS communication status, start and stop status, switch status, abnormal alarm and other status.

BMS monitoring: Meet the parameters and limit settings of the battery management system; monitor the temperature, voltage, and current of the battery cells and battery clusters of the energy storage battery; and provide alarms for abnormal battery charging and discharging status, voltage, current, and temperature.